The present invention is directed to a continuous processing system for performing high speed production operations on a plurality of unit products and, more particularly, to a continuous processing system using product accumulator inventory measurement for controlling processing machine speed in a loosely coupled mechanical system.
Loosely coupled mechanical systems are those in which discrete processing machines are coupled together to form a continuous process. The machines are generally coupled together by an accumulator connected between the machines. The accumulator provides a product flow path between the processing machines and also provides space for accumulating a product inventory between the machines.
An accumulator product "inventory" or "accumulation" as used herein refers to the number of unit products in the accumulator at any particular time and includes products flowing freely through the accumulator, hereinafter sometimes referred to as the "floating accumulation," as well as products positioned in relatively tightly paced relationship adjacent the downstream processing machine, hereinafter sometimes referred to as the "continuous accumulation" or "continuous-fill-accumulation." Many high speed processing machines require a continuous, i.e. uninterrupted, supply of unit products for proper operation. If there is a gap in the supply of unit products to such machines, incorrectly formed parts or faulty machine operation may occur. Thus when such machines are used in a loosely coupled mechanical system it is important to provide a continuous accumulation of relatively tightly packed unit products in a portion of an accumulator immediately upstream of such a processing machine to ensure that there will be a continuous supply of products to the machine. Monitoring or measurement of this continuous accumulation is necessary in high speed operations. A sufficiently large continuous accumulation must be maintained to provide sufficient time for the system to react, i.e. to provide enough time to enable the downstream machine to be slowed or stopped before the continuous accumulation is exhausted. For example, the downstream machine must be slowed or stopped if the flow of unit products into the accumulator upstream of the machine is slowed or stopped for an extended period. The monitoring or measurement of the continuous accumulation is also important to prevent too great a continuous inventory in the accumulator, i.e. an "overflow" condition. As overflow condition could cause damage to the upstream and downstream processing machines as well as to the products being processed.
Expressing this control situation metaphorically, the downstream machine needs to "know" if there is a sufficient continuous inventory to start processing or to increase processing speed, and the upstream machine needs to "know" if the accumulator is running out of area to store the unit products because of downstream stoppage or slow down. A problem in measuring product accumulation in an accumulator is caused by the fact that an accumulator has two areas with different product flow characteristics: a downstream area wherein the unit products are tightly packed, i.e. the continuous accumulation area, with product movement being dependent on downstream machine speed, and an upstream area where unit products are flowing freely through the accumulator unimpeded by downstream conditions, i.e. the floating accumulation area. The relative size of the floating accumulation area and the continuous accumulation area of an accumulator are generally constantly changing. An accumulator inventory monitoring system, to be effective, must be able to distinguish between free flowing products in the floating accumulation and tightly packed products in the continuous accumulation and must be able to quickly determine the number of unit products in the ever changing continuous accumulation.
One prior art inventory measuring system makes measurements of the continuous accumulation by placing sensors at discrete points in the accumulator and from the monitored continuous accumulation value determines the appropriate machine speed response. However, it take such sensors a relatively long period of time to distinguish between a unit product moving quickly (a product in the floating accumulation) and a unit product moving slowly or intermittently (a product in the continuous accumulation). Thus it takes such a system a relatively long time to update its "count" of the number of unit products in the continuous accumulation. As a result such a system has a slow reaction time and a very large accumulator is required for higher machine operating speeds because of the system's slow reaction time. An associated disadvantage of such a system is that no information is provided to the measuring system about the incoming unit product rate. The resolution of this type of system is limited by the number of sensors used and the response time of the sensors for differentiation between free flowing product conditions and tightly packed product conditions.
Another known inventory measuring method is counting the unit products entering the accumulator, counting the unit products leaving the accumulator and computing the number of products in the accumulator based on these counts. A disadvantage of this method is the necessary assumption that all unit products entering the accumulator are immediately available for processing. The count in/count out method does not account for accumulator delay, i.e. the time that it takes a unit product to move through the accumulator. Thus, although the total accumulator inventory may be calculated, the continuous accumulation cannot be determined by such a system. For machines that require a continuous supply of unit products such a system is of little value.